introduce BigInteger (type, reader and -str/-repr)
support most operations on bigintegers

missing: -quot, -rem and more importantly a proper divison. for that
we'd need to extend Ratio, but i'm not sure how we should do that. i
don't want to store pixie numbers in the ratio, so maybe it will be a
BigRatio. we'll see.
Merge pull request #48 from heyLu/bigints

Initial support for BigIntegers